Dredging operations are often needed for the improvement of a channel and/or harbour area.
Unavoidably the water resources close to the dredging site will be contaminated to some degree. This
paper initially presents the main feature and operation methods of new-work dredging and of
maintenance dredging. The former comprises the improvement (i.e. deepening or widening) of a
channel and/or harbour area mainly by removing stones and compacted sediments which were
deposited through geologic ages. The latter is employed mainly to remove the loose sediments which
tend to fill up previously excavated channels and harbours. The clamshell dredge, the dipper dredge, the
pipeline dredge, and the hopper dredge, are four common types of dredges that are used for river and
harbour excavation by the U.S. Army Corps of Engineers. Their functions, operations, and cost data, are
indicated.
In general only the sediments excavated from maintenance dredging operations are polluted with
obnoxious or toxic contaminants that are detrimental to the land, air, and water, environment. The
sources of pollution that are associated with the channel and harbour sediments may be categorized as
being from municipal and industrial discharge, storm-runoff, agricultural runoff, soil erosion, and
accidental spills.
Typical dredged materials from Ashtabula and Fairport Harbors in Ohio were collected and analyzed for
investigation. It was found that the sediments varied widely, in their physical and chemical
characteristics, between harbours and even at different locations within each harbour. Nevertheless,
the pollutants in the dredged materials of a specific harbour (or channel) can be generally characterized.
Settleability of the dredging samples was investigated in detail. It was observed that oil and grease and
heavy-metals were intimately associated with sediments and could be removed with settlement of
solids.
The current dredging practices in the U.S.A. includes the disposal of dredged materials in open sea, open
lake, along-shore diked areas, off-shore diked areas, abandoned strip-mine areas, and other disposal
sites. Due to the excellent settleability of the dredged materials, it is desirable that the polluted
dredgings be stored on a disposal site for some time prior to any treatment. A disposal area can be used
to create new land if satisfactory fill is produced. The satisfactory fill can be the non-polluted dredged
materials from the new-work dredging operations, or well-treated sediments from maintenance
dredging operations. The treated sediments or particles should be free from oil and grease, from toxic
heavy-metals, and from any substances which are putrescent, deleterious, or odorous. They should not
cause any organic decomposition or ground-water contamination.
The dredged materials can be separated into the sediment and the supernatant water (i.e., spoil) by
settling. From the dredging-settling data presented in this paper, it appears that sufficient settling of
dredging in a diked disposal area, followed by discharge of supernatant to Great Lakes water, will not
contravene the proposed Great Lakes Water Standards, and so the goal of water resources conservation
could be achieved.